1. Field of the Invention
The present invention relates to x-ray imaging, more particularly, to methods of producing laminography x-ray images.
2. Description of the Related Art
X-ray laminography is the process by which a flat plane (or, more recently, a single voxel) inside an object can be sharply imaged while all other planes inside the object are blurred. Laminography systems have existed for many decades. The original laminography x-ray systems were used to diagnose cancer in the lungs. The patient would lie on his back on a gantry with an x-ray film below him and an x-ray source above him. The source and the film were then moved simultaneously in a manner such that only one horizontal plane remained in focus, with all other planes being blurred. The process was then repeated to focus another plane.
In recent years, digital radiography has allowed the focusing of each of the planes simultaneously. However the introduction of computed tomography (CT) has replaced the use of laminography in most medical applications. One exception is breast imaging, where it is now rapidly being introduced. These systems typically have an x-ray source emitting a cone of x-rays above the breast and a two-dimensional (2D) detector array below the breast. The patient's breast and the 2D detector remain fixed while the x-ray source moves along an arc above the detector. The x-rays impinge on the detector successively through several angles to the normal to the detector from approximately +/−30°. The reason for not using larger angles is the difficulty of arranging the components of the system around the patient. The data is reduced typically using a complex array of electronics and successive approximations of a maximum likelihood algorithm.
CT, however, has not replaced the majority of conventional transmission x-ray inspection machines in airports and other applications. This is mainly because of the high cost of the CT technology and also the weight and size of the systems.
There is a major difference between CT imaging and laminography imaging. Present day CT technology requires that the x-rays penetrate each voxel in the object from at least 180°, and preferably 360°, within a single flat plane, producing a single slice image. The process is then repeated for each plane to image the entire object. Laminography imaging does not have this limitation. It merely requires that all of the x-rays that traverse a chosen voxel in the object are collected from as many angles as possible (in three dimensions) and that only this data is used to determine the image of this voxel. This procedure is repeated to image every voxel in the object.
Laminography imaging is improved substantially by using a greater number of angles from a larger number of directions in three dimensions. A paucity of angles and the inability to produce views of the voxel from angles that differ greatly from one another are the principal factors in causing streaks and other problems in laminography images.